Geoscience Reference
In-Depth Information
(UVB) rapidly attenuates in water (penetration
depth is of the order of millimetres) so protection
to UVB had to increase considerably. The optical
properties of plants (especially the effects of UV
radiation on plants) have been intensively studied,
partly in reaction to the recognition of the polar
ozone holes in the atmosphere (Rozema et al.
2002a; de Bakker et al. 2005; Pf¨ndel et al. 2006).
In land plants, UV absorption is achieved by aro-
matic compounds such as the flavonoids and their
derivatives (I-V) and other compounds produced
via the phenylpropanoid pathway such as hydroxy-
cinnamic acid (VI) and lignin. A wide diversity of
flavonoids are already present in the Bryophyta.
Flavonoids are absent in Hornworts (Stafford
1991; Rausher 2006) and have been found in one
alga, the Charophyte Nitella (Markham & Porter
1969; Iwashina 2009). Flavonoids, or their deriva-
tives in sediments, are of potential interest to eluci-
date the early terrestrialization of the embryophytes.
Flavonoids were widely used during the 1970s
for plant systematic as well as evolutionary studies,
in particular for angiosperms, based on the distinc-
tion of 'advanced' v. 'primitive' characters (Craw-
ford 1978; Giannasi 1978; Stuessy & Crawford
1983) (Fig. 2). However, it appeared that the
advanced v. primitive distinction in flavonoids
composition was not straightforward, and that one
given compound could be synthesized via different
pathways. Flavonoids may therefore not be fully
reliable indicators for phylogenetic studies at
higher taxonomic levels (Crawford 1978; Giannasi
1978). Flavonoids are known from Tertiary sedi-
ments such as Kaempferol (III) (Niklas & Giannasi
1977a, b) and their earliest record is of the biflavo-
noid 5-O-Methylginkgetin (II) from Cretaceous
Ginkgo fossils (Zhau et al. 2006). Accordingly,
despite their potential interest for the terrestrializa-
tion process, it seems that they are not preserved
long enough in sediments to provide further
insight into the early embryophyte evolution.
Suberin
Suberin (Tegelaar et al. 1995) is another ether-based
macromolecule produced by plants. It seems to
be primarily used to form barriers between com-
partments or with the exterior. Depending on the
place of deposition in the plant it protects against
fire, desiccation and pathogens and limits ion trans-
port and gas diffusion. It occurs in roots and tubers,
bundle sheet cells of C4 plants and as cork in woody
species that have secondary thickening (Enstone
et al. 2002; Franke & Schreiber 2007).
The biopolymer suberin consists of an arom-
atic domain. Aromatic building blocks are gen-
erated via the phenylpropanoid pathway such as
p-coumaric, ferulic and sinapic acids (VII-IX) also
found in sporopollenin and as alcohols in lignin
(see below), as well as an aliphatic domain with
aliphatic building blocks similar to those of the
plant cuticle (discussed above). The aliphatic
component is considered to reduce transport
whereas the aromatic part has been suggested
to inhibit pathogen invasion (Kolattukudy 2001;
Bernards 2002; Franke & Schreiber 2007). Like
lignin, suberin is not known from the most primitive
embryophytes. Being ester cross-linked, suberin
3-OH-
anthocyanidins
Pterocarpans
D
C
Flavan-3-ols
3-OH-PAs
Flavan-3,4-diols
B
Flavonols
Isoflavones
3-deoxy-
anthocyanidins
A
3-OH-flavanones
Biflavones
Flavan-4-ols
Flavones
Flavanones
Fig. 2. Evolutionary scheme of the biosynthesis of the major subgroups of flavonoids with a 5,7-dihydroxy A-ring.
Four levels, A, B, C, and D are shown. Levels A, B are found in bryophytes, C in ferns and fern allies and D in
gymnosperms and angiosperms. PA: proanthocyanidin (modified after Stafford 1991). See supplementary information
for molecular structures (structures I-V).
